docs: add a CONTRIBUTING.md (#659)

CONTRIBUTING.md

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+## Setting up the environment
+
+This repository uses [`yarn@v1`](https://classic.yarnpkg.com/lang/en/docs/install/#mac-stable).
+Other package managers may work but are not officially supported for development.
+
+To setup the repository, run:
+
+```bash
+yarn
+yarn build
+```
+
+This will install all the required dependencies and build output files to `dist/`.
+
+## Modifying/Adding code
+
+Most of the SDK is generated code, and any modified code will be overridden on the next generation. The
+`src/lib/` and `examples/` directories are exceptions and will never be overridden.
+
+## Adding and running examples
+
+All files in the `examples/` directory are not modified by the Stainless generator and can be freely edited or
+added to.
+
+```bash
+// add an example to examples/<your-example>.ts
+
+#!/usr/bin/env -S npm run tsn -T
+…
+```
+
+```
+chmod +x examples/<your-example>.ts
+# run the example against your api
+yarn tsn -T examples/<your-example>.ts
+```
+
+## Using the repository from source
+
+If you’d like to use the repository from source, you can either install from git or link to a cloned repository:
+
+To install via git:
+
+```bash
+npm install --save git+ssh://git@github.com:openai/openai-node.git
+```
+
+Alternatively, to link a local copy of the repo:
+
+```bash
+# Clone
+git clone https://www.github.com/openai/openai-node
+cd openai-node
+
+# With yarn
+yarn link
+cd ../my-package
+yarn link openai
+
+# With pnpm
+pnpm link --global
+cd ../my-package
+pnpm link -—global openai
+```
+
+## Running tests
+
+Most tests will require you to [setup a mock server](https://github.com/stoplightio/prism) against the OpenAPI spec to run the tests.
+
+```bash
+npx prism path/to/your/openapi.yml
+```
+
+```bash
+yarn run test
+```
+
+## Linting and formatting
+
+This repository uses [prettier](https://www.npmjs.com/package/prettier) and
+[eslint](https://www.npmjs.com/package/eslint) to format the code in the repository.
+
+To lint:
+
+```bash
+yarn lint
+```
+
+To format and fix all lint issues automatically:
+
+```bash
+yarn fix
+```
+
+## Publishing and releases
+
+Changes made to this repository via the automated release PR pipeline should publish to npm automatically. If
+the changes aren't made through the automated pipeline, you may want to make releases manually.
+
+### Publish with a GitHub workflow
+
+You can release to package managers by using [the `Publish NPM` GitHub action](https://www.github.com/openai/openai-node/actions/workflows/publish-npm.yml). This will require a setup organization or repository secret to be set up.
+
+### Publish manually
+
+If you need to manually release a package, you can run the `bin/publish-npm` script with an `NPM_TOKEN` set on
+the environment.